The aim of this article is to show the possibilities of supplementary heating of
greenhouses. There was used for this research an electric infrared heating panel ITA 700. The
average total power was 630.8 W in laboratory measurements, of which 504.3 W has been
transferred by the front part of the panel, of which 267.2 W has been transmitted by radiation.
The total radiation efficiency of the front part was 42.4%. Similar measurements have been
carried out in an experimental greenhouse (length 24 m, width 3.5 m). Approximately 448 W of
average total power 603.7 W has been transferred by the front part of the panel, of which 159 W
has been transferred by radiation. The total radiation efficiency of the front part was 26.3%.
Differences between measured surface temperatures confirmed the influence of panel radiation
on the flower bed. The average temperature in the soil (9 °C) shows that the influence of heating
is mainly on the surface of the flower bed, where it could protect cultivated plants during the
lowest temperature period. The obtained results can be useful for choosing suitable panel
parameters for the placement in small horticultural or hobby greenhouse

Kic, P.

English

EMU DSpace

2069Agronomy Research 18(3), 2069 2075, 2020https://doi.org/10.15159/AR.20.190Electric infrared heating panels as an alternative source ofheating for greenhousesP. KicCzech University of Life Sciences Prague, Faculty of Engineering, Department ofTechnological Equipment of Buildings, Kamycka 129, CZ165 21 Praha 6, Czech RepublicCorrespondence: kic@tf.czu.czAbstract. The aim of this article is to show the possibilities of supplementary heating ofgreenhouses. There was used for this research an electric infrared heating panel ITA 700. Theaverage total power was 630.8 W in laboratory measurements, of which 504.3 W has beentransferred by the front part of the panel, of which 267.2 W has been transmitted by radiation.The total radiation efficiency of the front part was 42.4%. Similar measurements have beencarried out in an experimental greenhouse (length 24 m, width 3.5 m). Approximately 448 W ofaverage total power 603.7 W has been transferred by the front part of the panel, of which 159 Whas been transferred by radiation. The total radiation efficiency of the front part was 26.3%.Differences between measured surface temperatures confirmed the influence of panel radiationon the flower bed. The average temperature in the soil (9is mainly on the surface of the flower bed, where it could protect cultivated plants during thelowest temperature period. The obtained results can be useful for choosing suitable panelparameters for the placement in small horticultural or hobby greenhouse.Key words: convection, heat transfer, low temperature, measurement, radiation, soil.INTRODUCTIONThe aim of this article is to show the possibilities of supplementary heating ofgreenhouses. Construction, calculation and design of heating systems in large scalegreenhouses are well described and studied in special publications e.g. Has, 2004;Pokluda & Kobza, 2011. The heating systems (hot water, electricity or gas)recommended for large scale greenhouses are usually very sophisticated, but veryexpensive and they are supposed to be used permanently during the long period e.g. inthe Czech Republic (autumn, winter and spring). Considerable attention is paid to theconstruction of greenhouses in terms of the use of progressive techniques (Avotins et al.,2018; Hart & Hartova, 2018).There are also used some simple constructions of greenhouses (mostly made fromplastics) used in many parts of the world. Constructions of these greenhouses are welldescribed in (Cermeno, 2002). There is no supposed heating. Small greenhouses, whichare used for growing seedlings, vegetables, flowers and other plants in the cold seasonunsuitable for growing in free garden, are usually without heating. These greenhouses,unlike large commercial greenhouses, are not equipped with a conventional overhead or2070underground heating system. A drop of outdoor air temperature and insufficient solarradiation e.g. in night, can cause plant destruction (spring frosts). That is the reason whysome scientific publications are focused on this issue. E.g. (Anifantis et al., 2017) carriedout an assessment of thermal energy with the use of small scale photovoltaic, hydrogenand geothermal stand-alone system for greenhouse heating.covered greenhouses is studied in (Tadj et al., 2014) as it greatly influences crop growthand development. It is focused on heating pipes and air heater. In conclusion, it is statedthat precise evaluation of the behaviour of heating equipment in a greenhouse isnecessary.One of solutions for heating in the critical period is the use of electric infraredheating panels. Infrared emitters have considerable applications in many sectors(Bulgakov et al., 2014). Unlike conventional convection heating panels, which first heatthe air and thus a lot of energy escapes (heat losses) through the surrounding walls, theelectric infrared heating panels transfer part of energy from their surface in the form ofradiation, i.e. directly on the plants and soil. This makes more efficient use of inputpower. The research work focused on the theory and construction of the electric heatingpanel is described in Greppi & Fabbri (2017). Unlike electric heating cables, which mustbe in the soil, these panels are easy to install.This research is focused on the verification of the function of electrical infraredlow-temperature panels in laboratory conditions and in an experimental greenhouse. Theenergy transfer and influence on the soil surface has been studied in real conditions.MATERIALS AND METHODSThere was used for this research an electric infrared heating panel ITA 700 withrated power 680 to 730 W, which is recommended to install on the wall or ceiling. Thefront part of the panel which is the principal source of heat transmission has surface areaSF = 0.75 m2 and the back part together with side parts (SO = 0.85 m2) are insulated andcovered with the aim to reduce the heat transfer through these parts. The panel was setThis paper includes the results of two types of experiments. There were carried outlaboratory experiments inside the Faculty of Engineering, which enabled to installheating panel and all instruments together for long distance measurement on the floor.The aim was to find out properties of heating panel in protected laboratory conditionswith average indoor temperature about 22 and relative humidity approximately 26%.Similar measurements have been carried out in an experimental greenhouse (length24 m, width 3.5 m and height 2.22 m) at outside air temperature about -2humidity approximately 65% and a solar radiation 520 W m-2. This greenhouse is notequipped with any heating system.InstrumentationAir temperatures and relative humidity were measured by data loggers ZTH65 withregistration at 1- minute intervals. Parameters of ZTH65 are: temperature operativerange 30 to +70 0.4 and operative range of relative humidity5 95% wi 2.5%.2071The surface temperatures of tested infrared heating panel were measured by specialsurface infra sensor FIA 260-MV with temperature operative range -18 to 260 with0.1 K. The surface temperatures in some specific places were measured byspecial surface sensor S 106 9R (NiCr-Ni, with adapter G017R) connected to theinstrument THERM 2253-2 (Ahlborn GmbH, Germany) for contact temperaturemeasurement. This instrument was used also with sensor for air temperaturemeasurement AMR TK 127 10R. This instrument and sensors can be used in operativerange from 100 to 1,370 , with display resolution 0.1 1%from the measured value.There were installed during the measurements surface thermocouples NiCr-Ni(type K) in operative range from 50 to +200 , with display resolution 0.1 and withaccuracTemperatures of the soil in the greenhouse were measured by the thermocouplesNiCr-Ni (type K) in operative range from 25 to +400 , with display resolution 0.1NiCr-Ni in operative range from 10 to +105 in the length installed according to theneeds of measurements.The heat flux density on the rear part of the panel was measured by special heatflux sensor FQA018C (Ahlborn GmbH, Germany) which has dimensions120 120 1.5 mm. It is made from epoxy resin with the resistance to 80 andcalibration constant 9.69 W m-2 mV-1 and relative measurement uncertainty 5%.Temperature and humidity of surrounding air has been measured by sensorFHA 646 21 including temperature sensor NTC type N with operative range from30 to +100 , and air humidity by capacitive sensor with1-minute intervals to measuring instruments ALMEMO 2590-9 and ALMEMO 2690.The measurement results were processed by Excel software and some of resultswere verified by statistical software Statistica 12 (ANOVA and TUKEY HSD Test).Theory and modellingTo evaluate the function and efficiency of the panel, it is necessary to assess itsenergy balance (Eq. 1), expressing the total power consumption of panel P:(1)where P total power consumption (W); PC power (heat) transmitted byconvection (W); PR power (heat) transmitted by radiation (W).Radiation heat transfer follows Newton-Richman law, Eq. (2). Basic principles andequations are described in many publications focusing on heat transfer, e.g. Sazima etal., 1989; Szekyova et al., 2006, which creates a theoretical background described in thenext part of this article.(2)where heat transfer coefficient (W m-2 K-1); S convective heat-transmittingsurface (m2); temperature difference between the panel surface and ambient air (K).2072The heat transfer coefficient depends on the Nusselt criterion Nu according toEq. (3):(3)where Nu Nusselt number; coefficient of thermal conductivity of air (W m-1 K- 1);L characteristic dimension (height) of the panel (m).For the vertical or inclined planar surface of the panel, Nu is determined by Eq. (4)depending on the Rayleigh's criterion Ry.(4)where C constant which depends on Ry; Ry Rayleigh's criterion; n exponent whichdepends on Ry.The Rayleigh's criterion Ry is determined according to Eq. (5):(5)where g gravitational acceleration (m s-2); expansion coefficient (K-1);kinematic viscosity of air (m2 s-1); a coefficient of thermal diffusivity of the air(m2 s- 1).The expansion coefficient is expressed by Eq. (6):(6)where tm characteristic temperature (K).(7)where tP surface temperature ( ); taThe total power transmitted by the panel can be divided into power PF transferredby the front part of the panel and power PO transferred by the other parts (back and side).(8)where PF power transferred by the front part of the panel (W); PO power transferredby the other parts of the panel (W).The power transfer efficiency F of the front part of the panel is calculatedaccording to Eq. (9):(9)where F power transfer efficiency of the front part of the panel (%).The efficiency of power transfer by radiation of the front part FR is calculatedaccording to Eq. (10):(10)where FR power transfer efficiency by radiation of the front part of the panel (%);PFR power transferred by radiation of the front part of the panel (W).2073RESULTS AND DISCUSSIONMain parameters measured in laboratory and greenhouse experiments are given inconsumption P was lower than was the nominal information. As results of the surfaceinsulation of back and side parts of the heating panel (SO = 0.85 m2) the powertransferred through these parts PO = 126.5 W is not too big and the power transferefficiency of the front part (SF = 0.75 m2) of the panel is rather high, in the laboratoryF = 80%, in the greenhouse PO = 155.7 W and F = 74.2%.Table 1. Results of power (W) and power transfer efficiency (%) (average and standarddeviation) measured and calculated in the laboratory and experimental greenhouseExperiments in P PO PF FLaboratoryGreenhouseSD standard deviation.Table 2 presents main results calculated for the power convection of the front partof the radiant heating panel. Final results of power calculation of the front part of theradiant heating panel in the laboratory and greenhouse experiments are presented inTable 3.Table 2. Results of Nusselt criterion Nu, Rayleigh criterion Ry and heat transfer coefficient(average and standard deviation) of the front part of the radiant heating panel calculated in thelaboratory and greenhouse experimentsExperiments in Ry NuLaboratory 7.83 109 109Greenhouse 9.27 109 1 109SD standard deviation.The average total power consumption was 630.8 W in laboratory measurements, ofwhich 20% has been transferred by the rear part of the panel and the remaining 504.3 Whas been transferred by the front part of the panel. It has been transmitted 237.1 W byconvection to the ambient airand 267.2 W by radiation mainlyto the floor.Similar measurementscarried out in an experimentalgreenhouse show thatapproximately 25.8% of averagetotal power 603.7 W has beentransferred by rear and side panelparts, and remaining 448 W hasTable 3. Results of power and power transfer efficiency(average and standard deviation) of the front part of theradiant heating panel calculated in laboratory andgreenhouse experimentsExperimentsinLaboratoryGreenhouseSD standard deviation.been transferred by the front part of the panel. It has been transferred by convection289 W to ambient air and 159 W by radiation mainly to the soil surface.2074The efficiency of power transfer by radiation of the front panel part are ratherdifferent (in the laboratory F = 42.4% and F = 26.3% in the greenhouse). It can beexplained by bigger heat transfer coefficient and higher percentage of the heattransferred by the convection in greenhouse.Table 4 presents measured surface temperatures in 2 m, 4 m and 6 m distance fromthe installed heating panel in the laboratory as well as in the greenhouse experiment. Thestatistically significant differences between these temperatures are caused by increaseddistance from the panel.Table 4. Results measurements of soil temperature at 5 cm deep in flower bed of greenhouse andsurface temperatures 2 m, 4 m and 6 m distance from heating panel (average and standarddeviation) measured in the laboratory and greenhouse experiments. Different letters (a, b, c) inthe superscript are the sign of high significant difference ( )between the surface temperatures of floor (laboratory) or soil surface in the greenhouseExperiments in tsoil ts2 ts4 ts6Laboratory - a b cGreenhouse 4.6a b cSD standard deviation.The surface temperature of the floor in the laboratory measured at a distance of 2 mfrom the panel was 23.0 m about 22.5 m about 22.3temperature of the soil in the greenhouse measured at a distance of 2 m from the panelwas 30.3in greenhouses than in the laboratory are caused by the influence of solar radiation(520 W m-2) and greenhouse effect. The average soil temperature at 5 cm deep was 9CONCLUSIONSThe measurement results show the possibility of using a radiant low-temperatureelectric panel as a possible source of greenhouse heating, but the total output power waslower than nominal.The total radiation efficiency of the panel front part was approximately 42.4%during the laboratory experiments. In similar measurements carried out in anexperimental greenhouse total radiation efficiency of the panel front part was about26.3%.The energy transferred through the rear and side panels and the energy transferredby convection of the front panel part are also brought to the greenhouse and used to heatthe indoor air. So, it has also positive impact on the indoor greenhouse conditions.Differences between surface temperatures confirm the influence of panel radiationon the flower bed. The average temperature in the soil shows that the influence of heatingis mainly on the surface of the flower bed, there it could protect cultivated plants, leavesand flowers during the lowest temperature period.The obtained results can be useful for choosing suitable panel dimensions andparameters for the placement in small horticultural or hobby greenhouse. The futureresearch could be focused on the long-time experiments and application in specialgreenhouses.2075ACKNOWLEDGEMENTS. The author wishes to express his thanks to the employees of theCULS Prague Garden Centre, who made it possible to perform measurements in an experimentalgreenhouse.REFERENCESAnifantis, A.S., Colantoni, A. & Pascuzzi, S. 2017. Thermal energy assessment of a small scalephotovoltaic, hydrogen and geothermal stand-alone system for greenhouse heating.Renewable Energy 103, 115 127.Avotins, A., Potapovs, A., Apse-Apsitis, P. & Gruduls, J. 2018. Crop weight measurement sensorfor IoT based industrial greenhouse systems. Agronomy Research 16, 952 957.Bulgakov, V., Bandura, V., Arak, M. & Olt, J. 2018. Intensification of rapeseed drying processthrough the use of infrared emitters. Agronomy Research 16, 349 356.Cermeno, Z.S. 2002. Construction of greenhouses. Ediciones Mundi-Prensa, Madrid, Barcelona,Mexico, 499 pp. (in Spanish).Greppi, M. & Fabbri, G. 2017. Thermal characterization of radiative and convective heatingdevices. Energy Procedia 126, 74 81.Hart, J. & Hartova, V. 2018. Development of new elements to automatized greenhouses.Agronomy Research 16, 717 722.Has, S. 2004. Greenhouses, characteristics and equipment. UZPI, Praha, 56 pp. (in Czech).Pokluda, R. & Kobza, F. 2011. Greenhouses, plastic greenhouses, utilization and growingtechnologies. Profi Press s.r.o., Praha, 253 pp. (in Czech).Sazima, M., Kmonicek, V., Schneller, J., Ambroz, J., Bayer, Z., Cerny, V., Hlavacka, V.,Pikman, M., Streda, I. & Sifner, O. 1989. Heat. SNTL, Praha, 588 pp. (in Czech).Szekyova, M., Ferstl, K. & Novy, R. 2006. Ventilation and air-conditioning. JAGA, Bratislava,359 pp. (in Czech).Tadj, N., Bartzanas, T., Fidaros, D., Draoui, B. & Kittas, C. 2010. Influence of heating system ongreenhouse microclimate distribution. Transactions of the ASABE 53(1), 225 238.

Electric infrared heating panels as an alternative source of heating for greenhouses

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Electric infrared heating panels as an alternative source of heating for greenhouses

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